Pokémon Go added the Vivillon family, a caterpillar-into-butterfly Pokémon that has different forms depending on where you catch it. The key to collecting Vivillon (and its different forms) is to send and receive postcards with your friends.
Scatterbug is the base Pokémon, and it requires 25 candy to evolve into Spewpa, which requires 100 candy to evolve into Vivillon.
To find Scatterbug, you need to pin postcards from different locations. Pinning three postcards from the same area will allow you to catch a Scatterbug that will evolve into a specific Vivillon. You can pin your own postcards and the postcards you get from friends. Having friends in different countries will help you collect more types of Vivillon, but you only need one type to fill in the Pokédex entry for it.
To catch the Scatterbug, you’ll need to look at your Vivillon medal and tap the “catch!” button that appears. Remember to use a Pinap Berry to catch your Scatterbug to get more candy.
You can also tell which Vivillon your Scatterbug is going to turn into by looking its information screen.
Depending on where the postcard comes from, you’ll get a different Scatterbug and Vivillon pattern. Niantic provides this helpful map to show which locations give which patterns. You can access this map yourself via tapping the Vivillon medal on your player info screen or you can take a look below.
As of writing this, you cannot get shiny Scatterbug, Spewpa, or Vivillon, but we’ll update this guide when they become available.